1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same, and in particular, an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunction printer, which employs an electrophotography process.
2. Description of the Related Art
Up to now, an optical scanning apparatus is used for a laser beam printer (LBP), a digital copying machine, or a multifunction printer. In the optical scanning apparatus, a light beam optically modulated and emitted from a light source unit according to an image signal is periodically deflected by an optical deflector configured by, for example, a rotary polygon mirror. The deflected light beam is focused on a photosensitive recording medium (photosensitive drum) surface in the form of a spot by means of an imaging optical system having an fθ characteristic, and the surface is optically scanned to record an image.
Up to now, various optical scanning apparatus using a single imaging lens as the imaging optical system for the purpose of making the entire apparatus compact have been proposed (see Japanese Patent Application Laid-Open No. H10-48552). Further, up to now, various optical scanning apparatus have been proposed, in which a single imaging lens having a free curved surface and a single plane mirror are combined to an optical system provided between the optical deflector and the surface to be scanned, so as to turn back a light beam by the plane mirror, to thereby reduce a size of the entire apparatus (see Japanese Patent Application Laid-Open No. 2008-158415).
Japanese Patent Application Laid-Open No. H10-48552 provides an optical scanning apparatus in which, in order to image a light beam from a light source onto the surface to be scanned by the single imaging lens made of plastic, the imaging lens is formed into a suitable lens shape and suitably disposed, to thereby reduce a lens thickness to be suitable to be made of plastic.
In Japanese Patent Application Laid-Open No. 2008-158415, the imaging optical system includes the single imaging lens and the single plane mirror. A light beam passing through the imaging lens is turned back by the plane mirror and then passes through the imaging lens again in a direction reverse to an incident direction, to thereby enable an entire size of the optical scanning apparatus to be reduced. In the optical scanning apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-158415, the plane mirror is tilted in a sub-scanning direction to separate between an incident light beam and an exit light beam or separate between the exit light beam and the optical deflector, on the surface of the imaging lens which is located on the optical deflector side.
In recent years, an optical scanning apparatus is sought after in which light beam separation in the sub-scanning direction is sufficiently ensured on the surface of the imaging lens and an apparatus height may be suppressed to a low level even in a case where the plane mirror is tilted in the sub-scanning direction and in a case where a light beam enters the optical deflector in an oblique direction.